After installing F1 2010 from Steam it told me there was a patch via Games for Windows Live, which it successfully downloaded and performed as far as I can tell. Am I missing some other patch which corrects this pit stop bug, or was it never fixed?

Not necessarily, I've got the patch for my xbox and I still get screwed over sometimes.

For example at silverstone I was far enough behind the leaders so that when I was going to pull out of my pit box they were passing me and I had to wait, then I ended up waiting on all the tail enders, went in 4th came out 23rd.

The easiest way around this problem is to edit the lap you are due to pit on before you leave the pits for the race. e.g. If you are starting on Option Tyre and your scheduled stop is lap 7, change it to lap 8 or 9 yourself, and you should be pitting away from the majority of the AI cars!

I tried the advice in the thread I quoted: Just ignoring the pit in request is perfectly possible. Your race engineer just says "pit in this lap" after you cross the line instead of entering the pits, so your team is ready for you without any effort or thought needed.

Just stay out 1 more lap. Ignore the pit call. As soon as you enter the lap you were supposed to have pitted, it reopens the pit for you, no need to do anything. On the lap after the pit call I have so far never had more than 2-3 seconds wait.

It's pathetic that it's not been fixed, but since they bring you into the pits when your tyres are just starting to go off, there is definitely another lap of life in them so it's no big deal to wait.